Re: [PATCH v3] qga: fence guest-set-time if hwclock not available

[Michal Privoznik <mprivozn@redhat.com>]

On 12/5/19 2:12 PM, Cornelia Huck wrote:
> On Thu, 5 Dec 2019 14:05:19 +0100
> Philippe Mathieu-Daudé <philmd@redhat.com> wrote:
> 
>> Hi Cornelia,
>>
>> On 12/5/19 12:53 PM, Cornelia Huck wrote:
>>> The Posix implementation of guest-set-time invokes hwclock to
>>> set/retrieve the time to/from the hardware clock. If hwclock
>>> is not available, the user is currently informed that "hwclock
>>> failed to set hardware clock to system time", which is quite
>>> misleading. This may happen e.g. on s390x, which has a different
>>> timekeeping concept anyway.
>>>
>>> Let's check for the availability of the hwclock command and
>>> return QERR_UNSUPPORTED for guest-set-time if it is not available.
>>>
>>> Reviewed-by: Laszlo Ersek <lersek@redhat.com>
>>> Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
>>> Reviewed-by: Michael Roth <mdroth@linux.vnet.ibm.com>
>>> Signed-off-by: Cornelia Huck <cohuck@redhat.com>
>>> ---
>>>
>>> v2->v3:
>>>     - added 'static' keyword to hwclock_path
>>>
>>> Not sure what tree this is going through; if there's no better place,
>>> I can also take this through the s390 tree.
>>
>> s390 or trivial trees seems appropriate.
>>
>>>
>>> ---
>>>    qga/commands-posix.c | 13 ++++++++++++-
>>>    1 file changed, 12 insertions(+), 1 deletion(-)
>>>
>>> diff --git a/qga/commands-posix.c b/qga/commands-posix.c
>>> index 1c1a165daed8..0be301a4ea77 100644
>>> --- a/qga/commands-posix.c
>>> +++ b/qga/commands-posix.c
>>> @@ -156,6 +156,17 @@ void qmp_guest_set_time(bool has_time, int64_t time_ns, Error **errp)
>>>        pid_t pid;
>>>        Error *local_err = NULL;
>>>        struct timeval tv;
>>> +    static const char hwclock_path[] = "/sbin/hwclock";
>>> +    static int hwclock_available = -1;
>>> +
>>> +    if (hwclock_available < 0) {
>>> +        hwclock_available = (access(hwclock_path, X_OK) == 0);
>>> +    }
>>> +
>>> +    if (!hwclock_available) {
>>> +        error_setg(errp, QERR_UNSUPPORTED);
>>
>> In include/qapi/qmp/qerror.h we have:
>>
>> /*
>>    * These macros will go away, please don't use in new code, and do not
>>    * add new ones!
>>    */
> 
> Sigh, it is really hard to keep track here :( I just copied from other
> callers in this file...
> 
>>
>> Maybe we can replace it by "this feature is not supported on this
>> architecture"? (or without 'on this architecture').
> 
> This is not really architecture specific, you'd get this on any setup
> that does not have /sbin/hwclock.
> 
> Q: Is libvirt doing anything with such an error message from QEMU? Do
> we have the freedom to say e.g "guest-set-time is not supported" or so?
> Or is it beneficial to print the same error message for any unsupported
> feature?

No. Libvirt threats error messages as an opaque data. In a few cases we 
check for the class of the error and for instance issue a different 
command if class was CommandNotFound. You are free to change error 
messages as you wish.

Note that this is not true for HMP - there libvirt does some parsing to 
figure out the source of error. For instance:

https://libvirt.org/git/?p=libvirt.git;a=blob;f=src/qemu/qemu_monitor_text.c;h=9054682d608b13347880f36cacd0e023151322e6;hb=HEAD#l36

Michal